The blood of domestic animals contains about 18% protein and may be regarded as an exploitable protein reserve corresponding to about 6-7% of the lean meat in the animal carcass. In recent years, hygienic blood collecting equipment (Wismer-Pedersen, 1979) has been installed in slaughterhouses in a number of countries in order to harvest this protein reserve. The blood is then separated into a plasma fraction and an erythrocyte fraction by centrifugation and, currently, mainly the plasma fraction is utilized in the meat industry as an additive to various meat products such as sausages.
However, most of the protein content of the blood is present in the erythrocyte fraction which is at present mainly utilized as an animal feed additive since even minor quantities of erythrocytes in food products impart to these an undesirable dark colour and unpleasant odour and taste. It would therefore be desirable to decolorize the erythrocytes so that their protein may be utilized in food products.
Various attempts have been made to decolorize the erythrocyte fraction of blood by removing the heme moiety from the hemoglobin. Thus, Tybor et al. (1975) describe removing the heme group from hemoglobin by means of an acidified acetone solution, while Sato el al. (1981) and Autio (1983) disclose absorption of heme on carboxymethyl cellulose to remove the heme portion of the hemoglobin molecule. These, however, are quite expensive processes and have therefore enjoyed limited commercial success. Alternatively, oxidative degradation, e.g. by means of hydrogen peroxide, has been suggested by Bingold (1949), Oord and Wesdorp (1979) and Mitsyk and Osadchaya (1970). Oxidation of hemoglobin by means of hydrogen peroxide is a very efficient way of removing the heme, but when erythrocytes are used as the starting material, the hemoglobin is protected by the action of catalase which decomposes hydrogen peroxide to water. The use of hydrogen peroxide as an oxidising agent to destroy the heme moiety therefore requires inactivation of the catalase. This has previously been done by reacting the erythrocytes with hydrogen peroxide at a temperature of 50.degree.-70.degree. C. where the catalase becomes inactive. However, this also leads to complete coagulation of the globin (the protein moiety of hemoglobin after removal of the heme moiety) which thereby becomes devoid of any functional properties.
Danish Patent Application No. 5508/86 discloses a process for producing decolorized hemoglobin by mechanical opening of the blood cells, adjusting the pH to 1-2 by means of an acid and adding 1-5% by weight/volume of an oxidising agent, in particular hydrogen peroxide, in the presence of a carbohydrate derivative containing dienol groups, such as ascorbic acid. After oxidation, side products (i.e. cell debris and heme) are optionally removed followed by recovering the decolorized hemoglobin.
European Patent Application, Publication No. 148 114 discloses a process for producing decolorized hemoglobin by treating whole blood in the presence of a proteolytic enzyme at a pH of 3.5-4 to denature the globin chains and thus render the heme portion more accessible to the action of an oxidising agent. The amount of oxidising agent added to the blood is 1% by volume.
French Patent Application No. 2 452 255 discloses treatment of blood with a strong base or a mixture of an oxidising agent and an acid at a pH of about 6.4 followed by filtration.
U.S. Pat. No. 4,180,592 discloses decolorization of blood by treating blood with an excess, e.g. 3-6% by weight, of an oxidising agent such as hydrogen peroxide after which excess peroxide is removed by adding a further amount of blood.
According to the known processes for removing heme from hemoglobin, a relatively high amount of oxidising agent is employed to degrade the heme moiety. This results in oxidation of sulfur-containing amino acids in the protein moiety and hence a decreased solubility and nutritive value of the protein for use as a food additive. Oxidation of the sulfur-containing amino acids may substantially be avoided by adding a dienol group containing compound to the reaction mixture concomitantly with the oxidising agent. Alternatively, the globin may be partially denatured by means of a proteolytic enzyme so that a smaller quantity (about 1%) of oxidising agent is required. However, such compounds are rather expensive and therefore contribute to increasing the cost of the decolorized protein product.